During the silent nighttime our clever bodies cleanses themselves from toxins and slug products. The "garbage" finds it's ways to get out of your body, as one example; it mixes up with your saliva. So if you brush your teeth when waking up in the morning before you eat your breakfast you help your body to get rid of these toxins faster, instead of swallowing them again when eating breakfast. This is actually an ancient ayurvedic fact that I've recently learnt.